The present invention relates to an improved hoisting apparatus that, through the use of a dual diameter or tapered drums and opposite direction wire rope wrapping, reduces the shaft torque required to lift a load with respect to that seen in conventional hoisting systems. With differing drum diameter sections, the shaft torque for the same operation may be positive, zero or negative. This improved design, besides allowing for smaller drum rotating mechanical equipment, has wide applications in the hoisting and craning industry. One of the applications is for use as a container crane's trim, list, skew and snag protection system (TLSS).
Container cranes hoist containers with four individual wire ropes. For purposes of simplification in explanation, each wire rope runs to a corner of a lift beam connected to the container. By controlled take up and let out of these four wire ropes the operator of the crane can force a container to tilt in the x axis, the y axis or yaw about a vertical z axis. In the craning industry these motions are called list, trim, and skew. In aircraft terms, these would be termed limited roll right and left, limited pitch nose and tail, and limited yaw clockwise and counterclockwise. (TLS) By adjusting these motions a suspended container can be forced to align better as it is moved on and off a ship and on or off a truck.
A snag occurs when a hoist is lifting a lift beam at high speed and the lift beam hangs up in a ship's hold, or alternatively, when the lift beam fails to stop when it reaches the underside of the hoist trolley. Although there is a significant amount of stretch in long wire ropes, once a snag occurs, if the upward lift of the crane is not stopped, damage will occur. Stopping the upward motion of the lift beam is not immediate as the hoist machinery keeps turning by virtue of its own flywheel inertia. The rotating kinetic energy associated with that flywheel inertia must be converted to heat, elastic strain or deformation. A typical snag a event only lasts about 0.3 seconds. For this reason container cranes must be equipped with a fast acting snag system.
Numerous prior art systems have been devised for both TLS and for snag. Most of these incorporate hydraulic cylinders in some manner. The most popular system combines four individual cylinders to serve all four functions. With this type of system, the same cylinder that can adjust wire rope length to perform one or more TLS functions can also release the wire rope in a controlled manner when needed for snag events. As a cylinder releases the wire rope, hydraulic oil flows through a metered orifice heating the oil and thereby absorbing much of the hoist flywheel energy. One problem with such prior art system, is that while a small cylindrical stroke is enough for TLS adjustment, snag compensation requires a large cylinder stroke. The control sensitivity for combining these large and small strokes on the same cylinder results in a poorly operating system for all four functions. Even the speed for trim and list is incompatible with the super sensitivity needed to control skew. For that reason crane operators prefer to separate TLS systems from snag systems and want adjustable speeds for the TLS features. A secondary problem with such prior art systems is hydraulic oil. Hydraulic systems usually leak and require a considerable amount of maintenance.
Stand alone mechanical TLS systems are already available, but are more expensive than hydraulic systems that can serve the same function. The combination mechanical TLS and hydraulic snag is a solution, but is too costly to be popular.
The present invention is a TLSS system that incorporates a drum with at least two different diameter sections upon which the different ends of a lifting wire rope simultaneously spool on and off of as a function of counterclockwise and clockwise drum rotation. The wire rope rides around an equalizing sheave which is rotatably connected to another equalizing sheave around which one of the main crane wire ropes ride. Altering the drum wire rope length moves the duo sheave assembly and causes the main crane wire rope's vertical length to be altered. The differing TLSS drum diameters act to alter the amount of shaft torque required to adjust the main crane wire rope length and increases the number of drum rotations required to do so. By using various combinations of multiple drum regions with different drum diameters, a precise, fast acting mechanical TLSS system using conventional electric motors can be designed for a crane's specific configuration.
Henceforth, a tapered TLSS drum and opposite direction wire rope wrapping would fulfill a long felt need in the hoisting industry. This new invention utilizes and combines known and new technologies in a unique and novel configuration to overcome the aforementioned problems and accomplish this.